During the assembly of a multi-component semiconductor device, a semiconductor wafer section such as a die is typically attached to a wafer section support such as a lead frame, a ceramic support, or a printed circuit board. For example, a die attach adhesive such as a silver-filled epoxy paste is interposed between a die and a lead frame, then the lead frame is placed on a hot plate or another heat source for 2.5 hours or longer to cure the die attach adhesive. After this curing step the assembly is further processed, which includes attaching bond wires between the die and the lead frame, then encapsulating the die and a portion of the lead frame in plastic.
Increasing production output, which decreases the cost per device, is a continuing goal of process engineers. Adhesives which can be cured during the normal course of device assembly, or “in-line curable” adhesives, have been proposed. U.S. Pat. No. 5,475,048 by Jamison et al., issued Dec. 12, 1995 which is incorporated herein by reference in its entirety, describes a conductor-filled thermoset resin which can be cured in less than two minutes at a temperature of 200° C. or below. Other adhesives which have a rapid cure profile given adequate temperatures are available.
Equipment is also available which provides rapid curing of adhesives. For example, the Sync-Rite SC-3000 oven available from Phicom USA, Inc. of Cupertino, Calif. contains eight programmable stages and has a cycle time of more than 80 seconds for each lead frame. This is a considerable time improvement over prior curing methods which require up to 2.5 hours but adds to equipment costs, processing steps, and space requirements.
One problem that often occurs with rapid curing such as that desirable with in-line cure processes is voiding in the adhesive material which results from outgassing during the adhesive cure. Voids form in the adhesive material lying between the two surfaces being attached, for example between a die and a lead frame, which provides a poor attachment that can later fail during operation of the device.
Additionally, excessive stress can be introduced to the package during a rapid cure process due to the rapid attainment of elevated temperatures and subsequent return to ambient in combination with large differences in the coefficients of thermal expansion of the package components, which can result in an early failure of the package or the semiconductor die itself. The in-line oven cure solution described above can reduce problems such as voiding and stress but, again, requires additional space, adds processing steps, and increases equipment costs.
A method and structure for attaching a wafer section to a wafer section support which increases throughput would be desirable.